As sensors and actuators are normally not (and have not been) treated in academic curricula as a subject in its own right; many students and current professionals often find themselves limited in their knowledge and dealing with topics and issues based on material they may have never encountered. Until now. This book brings sensors, actuators and interfaces out of obscurity and integrates them for multiple disciplines including electrical, mechanical, chemical, and biomedical engineering. Real world cases, worked examples, and problem sets with selected answers provide both fundamental understanding and how industry develops sensor systems. Students and professionals from any of these disciplines will easily learn the foundational concepts and then be able to apply them to cross-discipline requirements. The idea is simple. A sensor system in general is made of three components: 1.Inputs (sensors) 2.Outputs (actuators) 3.Processor (the unit to which the inputs and outputs are connected and performs all, or the most, tasks needed to interface them). Sensors, Actuators, and their Interfaces focuses on the broad area of detection, outlining and simplifying the understanding of theory behind sensing and actuation. It is an invaluable textbook for undergraduate and graduate level courses, as well as a reference for professionals who were never afforded the opportunity to take an introductory course.

The five senses - vision, hearing, smell, taste, and touch - are universally recognized as the means by which humans and most animals perceive their universe. They do so through optical sensing (vision), acoustic sensing (hearing), chemical sensing (smell and taste), and mechanical (or tactile) sensing (touch). But humans, animals, and even lower level organisms rely on many other sensors as well as on actuators. Most organisms can sense heat and estimate temperature, can sense pain, and can locate a sensation on and in the body. Any stimulus on the body can be precisely located. Touching of a single hair on the body of an animal is immediately located exactly through the kinesthetic sense. If an organ is affected, the brain knows exactly where it occurred. Organisms can sense pressure and have a mechanism for balance (the inner ear in humans). Some animals such as bats can echolocate using ultrasound, while others, including humans, make use of binaural hearing to locate sounds. Still others, such as sharks and fish (as well rays and the platypus), sense variations in electric fields for location and hunting. Birds and some other animals can detect magnetic fields and use these for orientation and navigation. Pressure is one of the main mechanisms fish use to detect motion and prey in the water, and vibration sensing is critical to a spider's ability to hunt. Bees use polarized light to orient themselves, as do some species of fish. And these represent only a small selection of the sensing mechanisms used by organisms.

Beyond the natural senses and actuation in living organisms, sensing and actuation is almost exclusively a human activitywhose ultimate purpose is to improve our lives and our interactions with the universe. Sensors and actuators are ubiquitous in our lives, whether we are aware of them or not. But beyond industrial sensors, those that produce many of the products we use, keep our transportation moving, and watch over our safety, there are two types of sensors and actuators that merit separate attention. The first class of devices includes those used to improve and sustain our health. From artificial limbs and organs to implantable devices, robot-assisted surgery, medical tests, and the manipulation of tissue and cells, this class of sensors and actuators is an important part of our health system and, indeed, life. They include systems such as X-ray imaging, magnetic resonance imaging (MRI), computed tomography (CT or CAT) scans, ultrasound scanning, and robotic surgery systems. Still others, of a perplexing variety, are used to test for every conceivable substance and condition in the body. The second class of devices expands our knowledge of the universe around us and, hopefully, allows us to better understand the universe, ourplace in it, and ultimately to live in harmony with it. Sensing of the environment not only benefits us, but contributes to the environment itself and all organisms in it. Off the planet, sensors allow us to protect ourselves from radiation, the effects of solar flares, and maybe even to avoid catastrophic collisions with meteorites, but perhaps most of all, they satisfy our curiosity.

Temperature sensors are the oldest sensors in use (excluding the magnetic compass), dating back to the very beginning of the scientific age. Early thermometers were introduced in the early 1600s. Around the middle of the 1600s, the need for standards of temperature measurement were voiced by Robert Boyle and others. Shortly after, about 1700, some temperature scales were already in use, devised by Lorenzo Magalotti, Carlo Renaldini, Isaac Newton, and Daniel Fahrenheit. By 1742, all temperature scales, including the Celsius scale (devised in 1742 by Andres Celsius), but excluding the Kelvin scale, were established. Following the work of Leonard Carnot on engines and heat, Lord Kelvin proposed the absolute scale bearing his name in 1848 and established its relation with the Celsius scale. The temperature scales were further developed and improved until the establishment of the International Practical Temperature Scale in 1927, followed by further revisions to improve accuracy.

Optical sensors are those sensors that detect electromagnetic radiation in what is generally understood as the broad optical range - from far IR to UV. The sensing methods may rely on direct methods of transduction from light to electrical quantities such as in photovoltaic or photoconducting sensors or indirect methods such as conversion first into temperature variation and then into electrical quantities such as in passive IR (PIR) sensors and bolometers. The third method of sensing related to optics - sensors based on light propagation and its effects (reflection, transmission refraction), which will not be discussed here because the optical aspect is usually not the sensing mechanism, but rather an intermediate transduction mechanism. Nevertheless, the physics will be mentioned briefly for completeness.

The class of electric and magnetic sensors and actuators is the broadest by far of all other classes, both in numbers and types and the variety within each type. Perhaps this should come as no surprise since in a majority of cases a sensor exploits the electrical properties of materials and, with few exceptions, the requisite output is electrical. In fact, we could argue that even sensors that were not placed in this category belong here as well. Thermocouples exploit electrical effects in conductors and semiconductors - an electrical phenomenon. Optical sensors are either based on wave propagation, which is an electromagnetic phenomenon, or on quanta, which are measured through electronic interaction with the atomic structure of the sensor. It would take little to argue that this is an electric phenomenon. In terms of actuation, most actuators are either electrical or, more commonly, magnetic. This is particularly true of actuators that need to provide considerable power. However, for the sake of simplicity and to follow the basic idea of limiting the number of principles involved in each class of sensors, we will limit ourselves here to the following types of sensors and actuators.

The class of mechanical sensors includes a fairly large number of different sensors based on many principles, but the four groups of general sensors discussed here - force sensors, accelerometers, pressure sensors, and gyroscopes - cover most of the principles involved in the sensing of mechanical quantities either directly or indirectly. Some of these sensors are used for applications that initially do not seem to relate to mechanical quantities. For example, it is possible to measure temperature through the expansion of gases in a volume. The expansion can be sensed through the use of a strain gauge, which is a classical mechanical sensor. In this application an indirect use of a strain sensor is made to measure temperature. On the other hand, some mechanical sensors do not involve motion or force. An example of this is the optical fiber gyroscope, which will be discussed later in this chapter.

The term acoustics can mean sound or the science of sound. It is in the latter sense that it is used here. Acoustics thus covers all aspects of sound waves, from low-frequency sound waves to ultrasound waves and beyond to what are simply called acoustic waves. As a means of sensing and actuation, acoustic waves have developed in a number of directions. The most obvious is the use of sound waves in the audible range for the sensing of sound (microphones, hydrophones, dynamic pressure sensors) and for actuation using loudspeakers. Another direction that has contributed greatly to the development of sensors and actuators is the extensive work in sonar - the generation and detection of acoustic energy (including infrasound and ultrasound) in water, initially for military purposes and later for the study of oceans and life in the oceans, and even down to fishing aids. Out of this work has evolved the newer area of ultrasonics, which has found applications in the testing of materials, material processing, ranging, and medicine. The development of surface acoustic wave (SAW) devices has extended the range of ultrasonics well into the gigahertz region and for applications that may not seem directly connected to acoustics, such as oscillators in electronic equipment. SAW devices are important not only in sensing, especially in mass and pressure sensing, but also in a variety of chemical sensors.

In this chapter, chemical sensors that are most important from a practical point of view and the principles involved in chemical sensing are discussed. Included in this chapter are electrochemical sensors that convert a chemical quantity directly into an electrical reading and follow the definition for direct sensors. Sensors that generate heat and where heat is the sensed quantity are also discussed. These sensors, just like the thermo-optical sensors, are indirect sensors, as are the optical chemical sensors. Following these are some of the most common sensors, such as pH and gas sensors. Humidity and moisture sensors are included here even though their sensing is not truly chemical, but because the sensing methods and materials relate to chemical sensors.

In this chapter, concerns with the ranges below and above UV, characterized by ionization-that is, the frequency is sufficiently high to ionize molecules based on Planck's equation. The frequencies are so high (above 750 THz) that many forms of radiation can penetrate through materials, and therefore the methods of sensing must rely on different principles than at lower frequencies. On the other hand, below the IR region the electromagnetic radiation can be generated and detected by simple antennas. Thus we will also discuss the idea of an antenna and its use as a sensor and an actuator.

In this chapter we look at some additional aspects of sensors and actuators, aspects that could not have been discussed in conjunction with the principles of conventional devices. First, we discuss a class of devices called microelectromechanical systems (MEMS). The term MEMS relates more to the method of production of sensors and actuators, whereas the sensors and actuators themselves are some of the devices discussed previously as well as others. We discuss them here because they are unique not only in the methods used to produce them, but at least some of them can only be produced as MEMS. One can imagine an electrostatic actuator, at least in principle. But only as an MEMS device does it become a useful, practical device. Then there is the issue of scale of fabrication. Using techniques borrowed from semiconductor production, enhanced by micromachining techniques, it became possible to mass produce sensors such as accelerometers and pressure sensors, and actuators such as microvalves and pumps. Many of these devices have been developed for the automotive industry, but they have found their way into others areas, including medicine.

The purpose of this chapter is to discuss the general issues associated with interfacing and to outline the more general interfacing circuits the engineer is likely to be exposed to. However, no general discussion can prepare one for all eventualities and it should be recognized that there are both exceptions and extensions to the methods discussed here. For example, an A/D is a simple - if not inexpensive - method of digitizing a signal for the purpose of interfacing with a microprocessor. However, this approach may not be necessary, or may be too expensive, in some cases. A case in point: Suppose that a Hall element is used to sense the teeth on a rotating gear. The signal from the Hall element is an AC voltage (more or less sinusoidal) and only the peaks are necessary to sense the gears. In this case a simple peak detector, followed perhaps by simple signal conditioning may be adequate. An A/D converter will not provide any additional benefit and is a much more complex and expensive solution. On the other hand, if a microprocessor is used and an A/D converter is available onboard, it may be acceptable to use it for this purpose in lieu of adding circuitry.

In this paper, a microprocessor will be viewed as a stand-alone, self-contained single-chip microcomputer. For this to apply, it must have a central processing unit (CPU), nonvolatile and program memory, and input and output capabilities. A structure that has these can be programmed in some convenient programming language and can interact with the outside world through the input/output (I/O) ports. But there are other less obvious requirements. Clearly, for a self-contained system, the microprocessor must be relatively simple, reasonably small, and hence limited in most of its features-memory, processing power and speed, addressing range, and of course, the number of I/O devices it can interact with. Unlike computers, the designer must have access to most features of the microprocessor - the bus, memory, registers, and all I/O ports. In short, the microprocessor is a mere component with flexible features that the engineer can configure and program to perform a task or a series of tasks. The limits on these tasks are only two: the objective limitations of the microprocessor and the imagination (or capabilities) of the designer. For the purpose of this discussion, it is narrowed down to 8-bit microprocessors since these are some of the simplest and are commonly used in sensor/actuator systems, and because they are representatives of all microprocessors (16and 32-bit microprocessors are also in common use, but the principles involved in interfacing are essentially the same). Even within these there are a number of architectures being used.

Least square polynomials or polynomial regression is a method of fitting a polynomial to a set of data. Suppose we have a set of n points (xi, yi) to which we wish to fit polynomial of the form y(x) = a0 + a1x + a2x2 + .... amxm. (A.1) Passing a polynomial through a set of data means selection of the coefficients so as minimize, in a global sense, the distance between the value of the function y(x) and the values at the points y(xi). This is done through the least squares method by first writing the “distance”function: n XS = i=1 (yi - a0 - a1xi - a2xi2 - .... amxim)2. (A.2) To minimize this function we calculate the partial derivatives with respect to each unknown coefficient and set it to zero. For the kth coefficient (k = 0, 1, 2, ... , m) we write aS aak or n X= -2 xik(yi - a0 - a1xi - a2xi2 - .... amxim) = 0 (A.3) i=1 n Xxik(yi - a0 - a1xi - a2xi2 - .... amxim) = 0. (A.4) i=1 Repeating this for all m coefficients results in m equations from which the coefficients through am can be evaluated. We show here how to derive the coefficients for a first order (linear) and second-order (quadratic) polynomial least square fit since these are the most commonly used forms. We assume n data points (xi, yi) as above.

The thermoelectric reference tables for the most common thermocouples are shown below. For each type of thermocouple we show first the general polynomial, followed by the table of coefficients and by the explicit polynomials for both the direct and inverse use. The output of the direct polynomials is in microvolts (μν). Output of the inverse polynomials is in degrees Celsius (°C). The index 90 indicates the standard used (in this case the International Temperature Scale of 1990 [ITS-90]).

In the following we explore a few issues associated with integer and fixed point mathematics on microprocessors. We do not discuss floating point mathematics since floating point operations are rarely resorted to in interfacing in the context of 8-bit microprocessors.